linux-kernel - Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral requests NMI-safe

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Message-ID: <20250123084213.GG3808@noisy.programming.kicks-ass.net>
Date: Thu, 23 Jan 2025 09:42:13 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 30/39] unwind_user/deferred: Make unwind deferral
 requests NMI-safe

On Wed, Jan 22, 2025 at 02:52:57PM -0800, Josh Poimboeuf wrote:
> On Wed, Jan 22, 2025 at 03:24:18PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 21, 2025 at 06:31:22PM -0800, Josh Poimboeuf wrote:
> > > +static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *cookie)
> > > +{
> > > +	struct unwind_task_info *info = &current->unwind_info;
> > > +	bool inited_cookie = false;
> > > +	int ret;
> > > +
> > > +	*cookie = info->cookie;
> > > +	if (!*cookie) {
> > > +		/*
> > > +		 * This is the first unwind request since the most recent entry
> > > +		 * from user.  Initialize the task cookie.
> > > +		 *
> > > +		 * Don't write to info->cookie directly, otherwise it may get
> > > +		 * cleared if the NMI occurred in the kernel during early entry
> > > +		 * or late exit before the task work gets to run.  Instead, use
> > > +		 * info->nmi_cookie which gets synced later by get_cookie().
> > > +		 */
> > > +		if (!info->nmi_cookie) {
> > > +			u64 cpu = raw_smp_processor_id();
> > > +			u64 ctx_ctr;
> > > +
> > > +			ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);
> > 
> > __this_cpu_inc_return() is *NOT* NMI safe IIRC.
> 
> Hm, I guess I was only looking at x86.

:-), right so x86 is rather special here, the various RISC platforms are
what you should be looking at, eg. arm64.